With infrastructure costs escalating, it is becoming essential to monitor and/or evaluate the health of concrete structures so that timely maintenance can maximize their useful lives. The initial strength and the service life of concrete used in an infrastructure, e.g., a roadway or a bridge, is significantly affected by its moisture content from the time that it is placed onwards. Moisture and temperature are the primary drivers for the hydration of Portland cement, and are essential factors in the most prevalent deteriorative processes, such as damage due to freezing and thawing, alkali-aggregate reaction, sulfate attack or delayed Ettringite formation.
One of the most devastating, yet most common, forms of concrete degradation is corrosion of embedded reinforcing steel, which is driven by moisture and temperature as the pH is lowered. Swelling of alkali-silicate gels, driven by sodium and potassium, is an additional failure mechanism in concrete.
In normal concrete that has not been contaminated by deicing salts or sea salt, steel (iron) is protected against corrosion by the passive nature of the high pH environment characteristic of Portland cement concrete. (Porewater in hardened concrete is often modeled as a saturated calcium hydroxide solution with a pH of 13+.) This natural passivity is the reason that steel-reinforced concrete is a chemically stable combination of materials. This balance is upset, however, when contaminants such as deicing salts penetrate the concrete or when concrete carbonates, formed by the conversion of calcium hydroxide to calcium carbonate, as CO2 permeates the capillary pores. Many different tests, summarized in Table 1, are currently required to provide complete data throughout the life of concrete.
TABLE 1Tests of Concrete ConditionStage inconcreteproduction,construction,and serviceMoisturelifeTemperatureContentpHChloride concentrationStockpiles ofComputeMeasureEvaluate cementDetermine chlorideraw materialsrequirements foraggregatetype by pH-timeconcentration ofheated or chilledmoisture contentrecord, based onaggregates, mix water,water based onfor waterdiffering rates ofand admixtures forrequired concreteadjustment inCa(OH)2 release bycompliance withdeliverybatching process.C2S and C3S.building code maximumtemperature andMeasure pH of mixvalues (ACI 318, ACIstockpiledwater: affects222).materials temp.cement hydrationCheck for aggregatesCheck againstand admixturethat are contaminatedcommon problembehavior.with salt from marineof hot cement duedeposits.to insufficientCheck that suchcooling in theaggregates have beenmfg. Process.washed as required.Identify cementcomposition byadiabatictemperature-time(“Heat Signature”of cementitiouspaste).Mixing,Check forEstimate waterDocument high pHDetermine chloridetransport, andcompliance withcontent of freshof fresh concreteconcentration in freshplacingminimum andconcrete,for demonstratingconcrete (regardless ofconcretemaximumcurrently bysafety hazard andCl− source) fortemperatureAASHTOneed for protectioncompliance withspecificationmicrowave ovenbuilding code maximumrequirementstest, ACvalues (ACI 318, ACI(ACI 301, 305,Impedance, or222).306)nuclear methods.Quality control (Q/C)Check that thecheck to make sure thatmixturenon-chloride admixturescomposition is ashave been used whenspecified by fieldrequired. Q/C check tocalorimetry (“heatmake sure seawater notsignature ofused when prohibited.concrete”).
Stage inconcreteproduction,construction,and serviceMoisturelifeTemperatureContentpHChloride concentrationRecently castPredict settingMonitor rate ofMonitor rate ofconcrete.time fromsurface drying tocarbonation (CO2)(1–4 hours)concreteprevent ordiffusion intotemperature viaminimize plasticsurface of concretematurityshrinkagethat can result inprinciple.crackingdusting and makeCheck forMonitorconcrete surfacescompliance witheffectiveness ofunpaintable.maximum oractions taken to(Common problemminimum in-prevent moisturewhen temporaryplace concreteloss. (See ACIheaters are nottemp.308 Guide toexternally vented,specificationsCuringACI 306)(ACI 305, 306)Concrete, Ch. 4)Estimate rate ofevaporation viaevaporativecooling in aridenvironments.Estimate rate ofearly-age strengthgain via maturitymethod.RecentlyTemperature-timeContinue toContinuedhardenedrecord used tomonitormonitoring ofconcretepredict strengtheffectiveness ofcarbonation.(12 hours to 7via maturity.actions taken toObservedays)Check forprevent moistureeffectiveness of flycompliance withloss (See ACIash, ground blast-maximum or308 Guide tofurnace slag, silicaminimum in-Curingfume and otherplace concreteConcrete, Ch. 4)pozzolans intemperature specsreacting with and(ACI 305, 306).neutralizingCheck forCA(OH)2compliance withmaximumtemperaturedifferencerequirements,within or betweenadjacentmembers. (ACI207).Over usefulTemperature usedEvaluate waterEvaluate effect ofCompare to chloride ionservice life ofto infer rate ofvaporpH on steelconcentration thresholdconcrete.deteriorativetransmission andpassivity.values for initiation ofprocesses: alkali-suitability forMeasure depth ofcorrosion.aggregateapplication ofcarbonation (nowEvaluate effectivenessreaction, sulfateflooringdone withof electrochemicalattack, delayedmaterials.phenolphthaleinchloride removal.EttringiteEvaluateindicator).formation,effectiveness of(Arrhenius-typeexternal orexponentialinternal water-temperatureproofinginfluence).measures.Monitor internalEvaluatetemperature toeffectiveness ofestimate onset oftraffic-bearingfreezing or tomembranes anddeterminesealers, vaporfreezing point.barriers.Infer presence ofEvaluatefreezable watermoisture effectsfrom internalon alkali-temperature.aggregatereaction, sulfateattack anddelayedEttringiteformation,freezable waterfor frost damage.
Hardened concrete has long been evaluated by laboratory wet analysis of extracted samples. While these techniques have been widely accepted, they are time-consuming and expensive. Electronic moisture sensors have been available for a considerable time, but have inherent limitations when embedded within concrete. The highly alkali environment can damage their electronics, and they are sensitive to electromagnetic noise. A further drawback of this approach is that power must be provided from an external source, requiring cables and connectors (e.g., Structural Health Sensor, Strain Monitor Systems, Inc., San Diego, Calif. 92101), or from an internal battery (e.g., Wireless Concrete Maturity Monitor, International Road Dynamic, Inc. Saskatoon, Saskatchewan, Canada). This necessarily increases the sensor's size and decreases the sensor's useful life.
Fiber-optic techniques have been proposed as an alternative means of evaluating concrete degradation. Several different techniques using optical fibers have been demonstrated for measuring moisture content. These techniques fall into two broad categories: (i) intensity measurements that take advantage of changes in light absorption as a function of moisture content, and (ii) measurements that rely on a change in the index of refraction. Within these categories, both single-mode and multi-mode fibers have been used, and various means to separate temperature and humidity effects have been employed. By using Time Domain Reflectography (TDR), multiple sensors have been monitored on a single fiber. The drawback of this approach is that only up to six sensors have been used. This somewhat decreases the cost of this type of sensor, but this is offset by the sophistication and cost of the equipment required for analysis.
Furthermore, these optical systems all have the drawbacks that they require laser sources and optical power meters for measurements, as well as sophisticated analysis, if multiple sensors are to be interrogated using TDR. If left in place, the equipment represents a considerable expense and can be vulnerable to environmental damage. If equipment must be carried to a site and connected to the fiber optic sensors, then set-up and analysis time can be lengthy, again adding to the expense.
Low-powered (e.g., nano-watt) electronic devices offer a possible solution, but in their current state of development, also have drawbacks. Even though they are low-powered, they have a limited life, particularly when they are powered by batteries.
2.1 Evaluation of Environmental Parameters in Concrete
Currently, concrete integrity or condition is evaluated primarily by extracting samples for wet chemical analysis in the laboratory where environmental parameters in the concrete, e.g., its moisture content, pH and ionic concentrations (e.g., concentrations of chloride, sodium or potassium ions) can be measured. Concrete samples are typically obtained by time-consuming and expensive methods, e.g., either pulverizing pieces cut form the hardened concrete, or by collected the dust from drilling holes in the concrete to prescribed depths.
Another environmental parameter, temperature, is evaluated in fresh concrete by ASTM C1064 (Standard Test Method for Temperature of Freshly Mixed Portland Cement Concrete). Evaluating the temperature of hardened concrete, such as recording the temperature-time history to estimate concrete properties as a function of the maturity, is most conveniently done by embedded thermocouples connected to data loggers. Pre-programmed data loggers known as “maturity meters,” are often used.
The maturity method estimates the degree to which the Portland cement has been hydrated, by recognizing a simple linear relationship between concrete temperature and the rate of cement hydration (known as the Nurse-Saul approach), or the more accurate non-linear Arrhenius reaction rate model proposed by Freiesleben-Hansen and Pederson. In either case, the normal assumption is that sufficient water is available to hydrate the cement, and moisture content data are not normally collected, nor are maturity estimates corrected when there is a lack of available water.
The drawback of the maturity method is that thermocouples are subject to corrosion and require instrumentation that is expensive and sensitive to the environment (and that therefore should not remain on-site). The maturity method is also an empirical model with considerable uncertanty, i.e., it gives only information on the state of hydration.
Moisture content of aggregates is currently determined by oven-drying techniques (conventional, microwave, or convection oven). Alternatively, electrical conductivity-based moisture meters may be permanently mounted in concrete batching plants. Another technique employs a “Speedy Moisture Meter,” which combines a wet or damp sand with a fixed amount of calcium carbide powder in a reaction chamber. A pressure gauge indicates the amount of acetylene gas generated, and thus infers the initial available moisture. The drawback of techniques that use oven-drying or moisture meters is that core samples must be taken from the concrete, which is time-consuming, expensive, requires skilled laboratory technicians, leaves openings that may not have been sealed, and which can allow water and salts ingress into the concrete.
Moisture content in hardened concrete can be estimated by removing samples of concrete and oven-drying them. Water-vapor transmission is estimated by collecting water on one face of the concrete (often with a desiccant such as calcium chloride.) Embeddable moisture gauges, also known as “internal humidity meters” have been built and used by several researchers, but are not routinely used (See ACI 308 Guide to Curing Concrete, Chapter 4, “Monitoring Curing Effectiveness.”)
pH measurements in fresh concrete were conducted by Thomas and Hover at Cornell in 1988 to evaluate the rate of Ca(OH)2 production due to cement hydration, using a conventional electronic pH meter. The drawback of this approach is that the viscous cement paste tended to clog the tip of the pH probe, making it difficult to record the continuously changing pH.
In hardened concrete, the most common pH evaluation technique is to spray phenolphthalein indicator on a concrete surface that has been freshly exposed by drilling, cutting, or cracking. Concrete that does not produce the characteristic pink/purple color is considered to have become acidified (most typically by ingress of CO2). The drawback of this approach is that it requires core samples be taken from the concrete. Also, this technique is qualitative rather than quantitative, in that it merely indicates whether pH is basic or acidic.
Chloride is extracted from concrete for analysis by either distilled water or acid to obtain the water-soluble or acid-soluble chloride concentration. The solution is then analyzed by any of a number of wet-chemistry lab methods (See ASTM C1152/C114.) Again, the drawback of this approach is that it requires extraction of chloride from the concrete, which is time-consuming, expensive and involves complex wet-chemistry diagnostic tests.
Assessment of the corrosion of steel reinforcing bars in concrete is commonly carried out by Linear Polarization Resistance (LPR), half cell potential, macrocell current measurements, or resistivity tests.
2.2 Embeddable Concrete Sensors
Although the above-described techniques are in wide use, they are necessarily time-consuming and expensive. The obvious advantages of in-situ testing have led to the development of embeddable sensors.
One type of embeddable sensor currently available is a half cell. A half cell potential survey can be conducted of reinforced concrete structures known, or believed to be, suffering from corrosion, particularly due to chloride contamination. Half cells are also embedded in concrete to monitor the performance of cathodic protection systems for atmospherically exposed steel in concrete, although the size of existing devices can make this difficult. The drawbacks of this approach (which is generally used only in buildings) are size, complexity and cost of these devices, plus external power requirements and need for read-out.
Another type of embeddable sensor currently available is a fiber-based moisture sensor. Fiber-based moisture sensors have recently been developed based on the technology for monitoring strain in bridges using optical fibers with Bragg diffraction. Jones and Kharaz used a multi-mode fiber with evanescent absorption in a gelatinous cladding layer to detect changes in moisture content (Jones and Kharaz, “A distributed optical-fibre sensing system for multi-point humidity measurement,” Sensors and Actuators A 46–47 (1995) 491–493). They report that the reversible hydration of cobalt chloride within the gelatin significantly changed the absorption of light at a wavelength of 670 nm while absorption at 850 nm was nearly unchanged. By using these two wavelengths, the effects of temperature were removed from the measurement of moisture content and a resolution of <2% was achieved. Although the authors claimed a useful range of humidity from 20 to 80%, the results showed that the range can be increased to 100% albeit with a reduction in the resolution. The drawback of this technique, however, is that the fibers remain embedded but read-out requires a laser source, a detector, and electronics that are expensive and sensitive to the environment (and that therefore should not remain on-site). Connecting to optical fibers is difficult, owing to the precise alignment that is required. All of the fiber-optic techniques share these drawbacks.
Bariáin et al. measured moisture content using a refractometer comprising a single-mode fiber with a double tapered section that was coated with agarose gel (Bariáin, Matáas, Arregui and López-Amo, “Optical fiber humidity sensor based on a tapered fiber coated with agarose gel,” Sensors and Actuators B 69 (2000) 127–131). As the refractive index changed with increasing moisture, attenuation across the tapered section was reduced. Results were presented for a range of 30% to 80% relative humidity (RH) but the range may be extended with some loss of sensitivity. The RH of concrete, however, can approach 100% at times, which limits the usefulness of such a refractometer. Furthermore, sensitivity to temperature was not measured.
Jindal et al. used a moisture-sensitive core within a section of single-mode fiber to increase the evanescent absorption as the sensor is traversed by light (Jindal, Tao, Singh and Gaikwad, “A Long Range, Fast-responsive Fiber Optic Relative Humidity Sensor,” Optical Engineering 41, 5 (2002) 1093–1096).
An embeddable electronic device from Virginia Technologies, Inc. (Charlottesville, Va.) can measure LPR, open circuit potential (OCP), resistivity, chloride ion concentration [Cl-] and temperature. Its drawback, however is that it must be connected to a data logger that also provides the unit with power.
An embeddable sensor to measure resistivity within concrete is also available (The Applied Physics Lab, The Johns Hopkins University). The device is wireless and powered by an RF source that also stores data for subsequent processing and analysis. Its primary drawback, however, is that it is unable to measure other parameters of interest. It also has the drawbacks of large antenna size and requirements for a particular orientation.
2.3 Passive Sensors
The sensors described in Section 2.2 may all be described as active, requiring continuous power from a battery or an external source in order to function. Passive sensors that operate by changes in the capacitive or inductive elements have also been proposed. Ong and his colleagues have developed passive sensors to evaluate temperature, humidity and pressure, the complex permittivity of the surrounding medium and the concentration of CO2, NH3 and O2 (Ong, Zeng and Grimes, “A Wireless, Passive Carbon Nanotube-based Gas Sensor,” IEEE Sensors Journal, 2, 2, (2002) 882–88; Ong, Grimes, Robbins and Singh, “Design and application of a wireless, passive, resonant-circuit environmental monitoring sensor,” Sensors and Actuators A, 93 (2001) 33–430). These devices operate in the range of 5 to 25 MHz and rely on changes in impedance. The operating distance was typically limited to under 10 cm.
Varadan et al. used a Surface Acoustic Wave (SAW) device operating at 915 MHz to measure temperature (Varadan, Teo, Jose and Varadan, “Design and development of a smart wireless system for passive temperature sensors,” Smart Mater. Struct. 9 (2000) 379–388). This device does not rely on measuring the impedance at the RF transmitter but rather uses the delay time between the emitted and returned signals. A frequency modulated RF wave is transmitted to the sensor and the induced current is converted to an acoustic wave that traverses the surface of a lithium niobate wafer, is converted back into an electrical signal and re-radiated by the antenna. The delay time is a function of the temperature of the wafer and the system resolution is 0.33° C. Measurements can be made over a range of one to two meters.
Butler et al. used a change in the inductance of the sensor's antenna rather than a change in capacitance to measure strain (Butler, Vigliotti, Verdi and Walsh, “Wireless, passive, resonant-circuit, inductively coupled, inductive strain sensor,” Sensors and Actuators A, 102 (2002) 61–66). A coil antenna was deformed in-plane to change its inductance and shift the resonance frequency. In other respects it operates similarly to the devices previously described.
Milos et al. (Milos et al., “Wireless Subsurface Microsensors for Health Monitoring of Thermal Protection Systems on Hypersonic Vehicles”, in Advanced Nondestructive Evaluation for Structural and Biological Health Monitoring, Ed. T. Kundu, Proceedings of SPIE 4335 (2001) 74–82) successfully combined a passive sensor with an RFID chip to produce a device that signals when a certain temperature has been exceeded. Two capacitors were attached in parallel with the antenna with one of the capacitors connected by a fusible link designed to melt at 292° C. The shift in resonance frequency was used to read the device while its location was determined from its identification number. In another version, the fusible link causes the transponder bit-stream to be inverted when it is broken. A device using this second version has been commercialized by SRI International and used to identify heat-shield tiles on the space shuttle whose adhesive may have been damaged during re-entry.
Those devices that measure the change in resonance frequency offer good sensitivity to a wide range of important phenomena and can potentially be used as concrete sensors. It is unfortunate but inevitable that their operating range is short, however, owing to the rapid reduction in mutual inductance with separation.
2.4 RFID Systems
As discussed above, low-powered electronic devices for monitoring concrete have a limited life, particularly when they are powered by batteries. By contrast, so-called Radio Frequency Identification (RFID) devices combine a microchip with an antenna (the RFID chip and the antenna are collectively referred to as the “transponder” or the “tag”). The antenna provides the RFID chip with power when exposed to a narrow band, high-frequency electromagnetic field. Such a device can return a unique identification (“ID”) number by modulating and re-radiating the radio frequency (RF) wave. The basic design of an RFID chip is extremely simple, containing only basic modulation circuitry, a rectification bridge and non-volatile memory.
RFID systems are gaining wide use due to their low cost, indefinite life and the ability to identify parts at a distance without contact. RFID systems are in widespread use for asset management, inventory, pallet tracking, electronic tolls, livestock tracking, building access and automobile security. They first appeared in tracking and access applications during the 1980s. Since these are wireless systems that allow non-contact reading, they may be used in manufacturing and other hostile environments where barcode labels could not survive.
The simplest RFID system provides one-way communication between an RFID transponder (i.e., an RFID chip and antenna) and a transceiver. A dipole antenna or a coil, depending on the operating frequency, is connected to the RFID chip and powers it when current is induced in the antenna by an RF signal from the transceiver's antenna.
Within the United States, commonly used operating bands for RFID systems center on one of three government-assigned frequencies: 125 KHz, 13.56 MHz or 2.45 GHz. A fourth frequency, 27.125 MHz, has also been assigned. Of these frequencies, only the GHz range provides a true RF link, while the others operate as electromagnetic transformers.
When the 2.45 GHz carrier frequency is used, the range of an RFID chip can be many meters. While this is useful for remote sensing, there may be multiple transponders within the RF field. In order to prevent these devices from interacting and garbling the data, however, anti-collision schemes must be used.
Since an infrastructure may remain in place, or have a useable lifetime of a hundred years or more, there is a need in the art for an embeddable sensor for monitoring concrete condition that can survive for the lifetime of the concrete. Furthermore, there is a need in the art to provide embeddable concrete sensors that can be used to detect changes in environmental parameters in concrete such as moisture content, temperature, pH, and the concentration of ions (e.g., chloride, sodium and potassium ions). Such sensors could provide critical data for monitoring the condition of concrete, starting with the initial quality control period of freshly mixed or freshly cast concrete, through the concrete's useful service life, and through its period of deterioration and/or repair.
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.